Microlithography is used for producing microstructured components, for example integrated circuits. The microlithographic process is performed using a lithography system, which has an illumination system and a projection system. The image of a mask (reticle) illuminated via the illumination system is in this case projected via the projection system onto a substrate (for example a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection system, in order to transfer the mask structure to the light-sensitive coating of the substrate.
Driven by the desire for ever smaller structures in the production of integrated circuits, EUV lithography systems that use light with a wavelength in the range of 0.1 nm to 30 nm, in particular 4 nm to 6 nm, are currently under development. In the case of such EUV lithography systems, because of the high absorption of light of this wavelength by most materials, reflective optical units, that is to say mirrors, have to be used instead of—as previously—refractive optical units, that is to say lens elements. For the same reason, beam shaping and beam projection should be performed in a vacuum.
The mirrors may for example be fastened to a supporting frame (force frame) and be configured to be at least partially manipulable or tiltable in order to allow a movement of a respective mirror in up to six degrees of freedom, and consequently a highly accurate positioning of the mirrors in relation to one another, in particular in the pm range. This allows changes in the optical properties that occur for instance during the operation of the lithography system, for example as a result of thermal influences, to be corrected.
Further, mirrors are used in illumination systems for lithography systems. The mirrors used then are desirably positioned precisely and the situation or position thereof sensed. For the purposes of moving the mirrors, actuators which are actuated by way of a control loop are provided for the most part. An apparatus for monitoring the tilt angle of a respective mirror using sensors is then frequently provided as part of the control loop.
For example, WO 2009/100856 A1 discloses a facet mirror for a projection exposure system of a lithography system, which has a multiplicity of individually displaceable individual mirrors. To ensure the optical quality of a projection exposure apparatus, very precise sensing and positioning of the displaceable individual mirrors is involved. Furthermore, document DE 10 2013 209 442 A1 states that a field facet mirror can be in the form of a microelectromechanical system (MEMS).